The best reason to watch NBC’s “The Blacklist” — this season’s No. 1 new show — is star James Spader, who plays fugitive-turned-FBI-informant Raymond “Red” Reddington and chews the scenery every week with his sharp-tongued one-liners and strange intensity.

But driving the plot of this drama are the villains of Red’s titular “blacklist.” They have ranged from the Stewmaker, who turned his victims into chemical stew to erase all DNA, to Milton Bobbit, who recruited terminally ill patients to perform assassinations for hire.

“Blacklist” writers customize their creeps of the week — characters who wouldn’t already be on the FBI’s radar and whose proclivities are not already being seen on other network procedurals.

“Because Red has this Rolodex of criminals he can access, our show — unlike others which might be just about homicides or just about crimes in certain ways — we can end up in all these worlds,” creator Jon Bokenkamp tells The Post.

Most of the villains are dreamed up by the show’s writers, though sometimes they are inspired by real-life criminals, and Bokenkamp even hired some comic book writers to help stockpile ideas.

“A lot of our villains, as demented as they may be, there’s some sort of reasoning that is emotional and makes them not just a psychopath — though they may be that,” Bokenkamp says. “Something that makes them relatable in some way.”

Though each episode has focused on one criminal, Bokenkamp teases that a few will return in the two-part finale that starts Monday at 10 p.m. One of the episode’s major plot points is that the villains are not all as unrelated as they might appear.

“In the season finale, there’s some connective tissue that we’re going to expose with some of the Blacklisters,” he says. “Certainly not all of them. But we’ve always felt that it was a bit of a puzzle that we were putting together.”

The finale will also confront the eroding relationship of the two main characters after last week saw profiler Liz Keen (Megan Boone) end her partnership with Red after learning that he killed her adoptive father, Sam. The stories of his death and the fire that traumatized Liz as a child will be further explored, as will Red’s own fears.

Megan Boone as Elizabeth Keen.Photo: NBC

“One of the things that’s most interesting to me is what makes him potentially vulnerable,” Bokenkamp says. “We are going to be introducing a Blacklister in the season finale who will definitely give us a bit of insight into what makes Red tick and what he’s afraid of.”